Electrolytic production of lead



Patented Sept. 7, 1937 PATENT OFFICE 7 2,092,451 ELECTROLYTIC PRODUCTIONor LEAD Allan Robert Gibson, Redland, Bristol, and Stanley Robson,Shirehampton, Bristol, EnglamL assignors to The National smeltingCompany Limited, London, England, a limited-liability company oi.England No Drawing. Application October 12, 1935, Serial No. 44,804. InGreat Britain November 5, 1934 3 Claims. '(Cl. 204-19) This inventionrelates to the treatment 'of' metalliferous materials containing leadand sulphur.

It has been proposed hitherto to add a sulphide ore such as leadsulphide to a fused lead chloride bath and by electrolysis to recoverthe metal and elementary sulphur from the respective electrodes. We havefound that in order to obtain a constant production of elementary leadand sulphur with the least consumption of electrical energy, acomparatively low concentration of lead sulphide is generally necessary,but the concentration is dependent upon the temperature of the melt andthe current density applied.

According to the present invention lead and sulphur are separated frommetalliferous materials containing them (for example, galena) bysubmitting to electrolysis fused lead chloride containing the saidmetalliferous material, the process being characterized in that theconcentration of lead sulphide in the melt is maintained between 1% and10% and the current density is so adjusted for this range ofconcentrations that i as the concentration is increased above 1%, the

the melt of 3% at a temperature of 550 C. and

employing a current density of 600 to 750 amps. per square foot. If theprocess is conducted at the lower end of the ranges given above, 1. c.with a current density of 500 amps -per square font a temperature of 500to 600 C. and a galena concentration of 1 to 2%, electrolysis proceedsnormally. If, however, with the current density and temperatureremaining constant, the galena concentration be progressively increasedthe ohmic resistance of the melt is decreased until a point is reachedin the neighbourhood of 3% galena concentration at which normalelectrolysis is suspended and the sulphur is retained by the meltpresumably in the form of di-' or poly-sulphide of lead. Beyond thispoint if the galena concentration is increased the current density mustbe correspondingly increased until with a be about 750 amps. per squarefoot. with this increase of current density the temperature of the meltwill naturally rise. However, normal electrolysis of the cell mayberestored by cessation of the feed oi lead sulphide. Sulphur will then beliberated at the anode, and PbSreformed at the cathode by electrolyticdecomposition of the'complex, until eventually only lead sulphide (PbS)remains in themelt, when normal electrolysis with the formation of thelead at the cathode and sulphur vapour at the anode is resumed.

In one wayof carrying out the process of the invention lead chloride ismelted and maintained in the fused condition in any well known type ofcell suitable for the electrolysis of fused salts, and galena isadded-from time to time to maintain a concentration of about 3-5%. Thecurrent density employed for this concentration is .about 750 to 1000amps. per square foot of electrodes surface. We prefer to employ acolumn of bi-polar electrodes with a gap of A inch. The voltage dropover each gap is 1.2 to 1.4. The supply of galena to the cell isregulatedaccording to the voltage over the cell, the temperature beingconstant. At a fixed temperature the voltage over the cell serves as aguide to the concentration of galena in the melt andthis may bemaintained approximately constant by additions of galena duringoperation. If in this way too much galena is added it will also bereadily observed by the decrease or cessation of sulphur at the anodeowing to the formation of complex sulphides'in the melt, a conditionwhich may be remedied by cessation of the feed or increase of thecurrent density. It is desirable ,to employ mechanical agitation to keepthe composition of the electrolyte constant, because the agitation dueto thermal convection currents only is insufiicient for this purpose.Under these conditions lead is recovered with a consumption of 750 to800 kilowatt hours per ton of lead, and the sulphur is recovered in theelementary state.

According to a feature of the invention the mixture may be subjected toa preliminary electrolysis to remove metals electro-negative to lead, inparticular silver.

We have found it desirable to make the additions of galena to an innercompartment which can be sealed from the electrolysis proper, In thisinner compartment the gangue formed on the addition of galena may bereadily skimmed from the melt. The preliminary electrolysis may also becarried out in the inner compartment while it is sealed fromelectrolysis proper, or

purification may be effected by allowing the mixture to pass through amolten lead seal between the inner compartment and the cell proper.

Using bi-polar electrodes the passage of the current causes separationof molten lead at the cathode surfaces and gaseous sulphur at, or near,the anode. The sulphur is collected by means of a suitable refluxcondenser, and the lead is removed in the molten state from the cell bymeans of a suitably designed syphon.

The gangue removed from the melt will contain some lead chloride, andthis may be removed by volatilization of lead chloride and the fumecondensed on galena and returned to the cell.

What we claim is:-

1. A process for separating lead and sulphur from metalliferousmaterials containing lead sulphide which comprises submitting toelectrolysis fused lead chloride containing the said metalliferousmaterial with a concentration of lead sulphide in the melt between 1%and 10% and at a current density between 500 and 1000 amps. per squarefoot, the current density varying proportionately to the lead sulphideconcentration.

2. A process for separating lead and sulphur from metalliferousmaterials containing lead sulphide comprising submitting a solution of 3to 5% of lead sulphide in lead chloride to electrolysis at a currentdensity between 750 and 1000 amps. per square foot so as to causesulphur vapour to be given oil at the anode, adding further leadsulphide-containing material to the melt and regulating the rate ofaddition and the current density so that the flow of sulphur vapour fromthe neighbourhood of the anode remains substantially constant.

3; A process for separating lead and sulphur from metalliferousmaterials containing lead sulphide which comprises electrolyzing in acell employing bi-polar electrodes with a voltage drop of 1.2 to 1.4over each gap, 9. solution of 3% to 5% of lead sulphide in leadchloride, adjusting the current density between 750 and 1000 amps. persquare foot according to the concentration of lead sulphide so that asubstantially constant flow of sulphur vapour is maintained from theneighbourhood of the anode.

ALLAN ROBERT GIBSON. STANLEY ROBSON.

